Mirages appear because differences in air temperature cause light rays from an object to take different paths to a viewer’s eye. Warm air near the ground bends light, so when the light reaches the viewer’s eye, the ray seems to point into the ground. This produces a second image that looks like a reflection of the object.

The brilliance of diamonds is due to their high refractive index, a measure of how strongly a transparent material bends light rays. The skill of the gem-cutter lies in angling the facets of the stone so that each light ray entering it is reflected many times before it emerges again.

Optical devices such as mirrors and lenses reflect and refract (bend) light. These illustrations show how light from an object forms different images when it is reflected by a curved mirror or refracted by a lens.

A strand of fiber optic cable reflects the light that passes through it back into the fiber, so light cannot escape the strand. Fiber optic cables carry more information, suffer less interference, and require fewer signal repeaters over long distances than wires.

A magnifying glass is a large convex lens commonly used to examine small objects. The lens bends incoming light so that an enlarged, virtual image of the object (in this case a mushroom) appears beyond it. The image is called virtual because it is only perceived by the viewer’s brain, and cannot be produced on a screen.

The Fresnel lenses in this lighthouse tower are each a collection of multiple glass prisms that bend nearly all available light into a powerful central magnifying lens. Each central lens points in a different direction and can project light beams visible up to 45 km (28 mi) away.

Polarized light consists of individual photons whose electric field vectors are all aligned in the same direction. Ordinary light is unpolarized because the photons are emitted in a random manner, while laser light is polarized because the photons are emitted coherently. When light passes through a polarizing filter, the electric field interacts more strongly with molecules having certain orientations. This causes the incident beam to separate into two beams, whose electric vectors are perpendicular to each other. A horizontal filter absorbs photons whose electric vectors are vertical (as shown here). The remaining photons are absorbed by a second filter turned 90° to the first. At other angles the intensity of transmitted light is proportional to the square of the cosine of the angle between the two filters. In the language of quantum mechanics, polarization is called state selection. Because photons have only two states, light passing through the filter separates into only two beams.

X-ray diffraction has been a useful tool in understanding the structure of solids. The lattice of atoms in a crystal serves as a series of barriers and openings that diffracts X rays as they pass through. The diffracted X rays form an interference pattern that can be used to determine the spacing of atoms in the crystal. This precession photograph shows the pattern resulting from X rays passing through a palladium coordination complex, a compound with a palladium atom at the center of each molecule.

A curved piece of glass resting on a flat piece of glass creates colorful concentric bands, called Newton’s Rings. The rings are caused by light waves reflecting from the two surfaces and interfering, or combining, with each other. A band of color appears wherever the distance between the two pieces of glass causes light of that color to interfere constructively, making the waves bouncing off the two surfaces reinforce each other. At these places, light waves of other colors interfere destructively and tend to cancel each other out.